A common industrial process is the manufacturing of continuous sheets of material, such as paper and plastic, and in order to control the production process sheet-gauging apparatus are used. For example, U.S. Pat. No. 3,621,259 owned by Measurex Corporation teaches a sheet gauging apparatus which includes two beams, one located above the sheet and one located below the sheet. Gauging head assemblies are mounted on the beams and the assemblies include sources and detectors. The sheet gauging apparatus also includes means to reciprocate the gauging head assemblies along tracks carried by the beams to allow measurement of various properties of the sheet at different positions on the sheet. For example, in a paper manufacturing process the moisture content, thickness, basis weight and many other properties of the paper sheet can be measured. It is common in practice for the sheet materials which are being measured by the gauging apparatus to sometimes be quite hot. Radiant heat from the sheet and steam rising from the sheet can cause temperature gradients in the beams which can result in deflection or deformation of the beams and tracks. The deflection or deformation results in variability of the separation of the gauging head assemblies as they travel along the tracks across the sheet which can cause measurement errors.
To reduce deflection and deformation of the beams and tracks, the system taught in U.S. Pat. No. 3,621,259 includes two fans to draw cool air from outside the device and to cause the air to flow longitudinally along the tracks and beams to stabilize the temperature thereof along their entire lengths. However, in some cases this system does not sufficiently reduce deflection of the beams.
Certain techniques are also known to compensate for the variability of the space between the gauging heads. For example, U.S. Pat. No. 4,678,915 owned by Measurex Corporation teaches a device for providing such compensation by determining the separation of the head assemblies and correcting the measured values of the parameter of interest (for example, basis weight, moisture content, and the like) according to the separation of the head assemblies. However, for certain measurements it is desirable to minimize the compensation which is required.
Accordingly, it is an overall object of the present invention to provide an improved system for reducing the thermal deflection and deformation of the beam and tracks in a sheet gauging apparatus.